The present invention relates to methods for preparation of substantially round and spherical pellets comprising sintered titanium dioxide. The pellets are useful as a scouring media.
Methods disclosed herein involve mixing titanium dioxide dust, water and at least one surfactant, forming a pellet from the mixture and sintering the pellet to form the titanium dioxide scouring media.
The production of titanium dioxide through the vapor phase oxidation of titanium tetrachloride has become a commonly used process in the production of titanium dioxide pigment. During this process, titanium tetrachloride vapors react with oxygen or air or both at high temperatures in an oxidation reactor. The resultant stream of hot oxidation products exiting the reactor contains titanium dioxide in a finely distributed form suspended in gaseous constituents. The freshly formed titanium dioxide particles, which are present at first as very reactive particles, grow together to form small aggregates having a diameter of less than 0.15 mm. Some of these small aggregates grow larger in size and partly sinter to form larger, hard and unshapely aggregates having a dimension greater than 0.15 mm. These larger aggregates tend to form in a layer and deposit on the surfaces of the hot reaction zone of the oxidation reactor. Continued use of the oxidation reactor and the hot reaction products which pass over it constantly regenerate this layer. Not only does this layer regenerate with continued use of the reactor, but often pieces of this layer break off and are carried away in the reaction mixture. These large particles are not suitable for pigment production and serve to contaminate the reaction mixture.
To prevent the formation of this layer of titanium dioxide particles, scouring media consisting of chemically inert scrubbing solids have been added to the reaction mixture at appropriate locations. Numerous substances have been used as scouring media such as aluminum oxide, zirconium silicate, and silica sand. One of the most commonly used scouring media is silica sand. However, there are disadvantages to the use of silica sand as a scouring media in an oxidation reactor.
Silica sand is somewhat angular in shape. While the angular shape is effective in scouring the walls of the reactor, it also severely wears the inner walls of the reactor. Eventually, the silica sand generates holes in the reactor, which requires repair to, or in some cases replacement of, the reactor. The replacement time could be a matter of weeks, thus severely hampering production.
Furthermore, silica sand particles are easily crushed. The crushed silica sand contaminates the reactor mixture and must be removed from the mixture. If not removed, the silica sand will reduce the quality of the resultant product. Therefore, there exists a need for a scouring media that will be strong and dense enough to effectively scour the titanium dioxide reactor walls but is round and spherical and less abrasive to reduce or eliminate reactor wear. There is also a need for a scouring media that is sized properly to provide adequate separation and recycle and would reduce the potential for contamination of the reactor mixture.
Titanium dioxide has been used as a scouring media to address the disadvantages associated with the use of conventional scouring media. However, the titanium dioxide previously produced for use as scouring media has not eliminated all of the problems associated with other scouring media. For instance, in the process described in U.S. Pat. No. 4,784,841, titanium dioxide scouring particles are produced by vapor phase oxidation of titanium tetrachloride. The particle aggregates initially formed which have a dimension larger than 0.15 mm are separated, admixed with alkaline hydroxides, calcined and returned to the process. This process produces particle aggregates of sizes between 0.2 to 2.0 mm for use as scouring aggregates for the prevention of deposit formation on a heat exchanger. The process disclosed in U.S. Pat. No. 4,784,841, however, fails to control the size and shape of the aggregates formed, and thus fails to reduce the possibility of damage to the reactor walls and fails to maximize separation and recycle/reuse efficiency.
In the process described in U.S. Pat. No. 2,721,626, coarse scouring solids having a dimension ranging from 0.15 to 6.35 mm are admixed with a hot reaction mixture and after cooling separated from the reaction products and used again as scouring solids. The dry separation of the coarse scouring titanium dioxide particles from the finely particulate titanium dioxide is inefficient. Thus, there is always a percentage of coarse scouring solids remaining in the final product thereby reducing the overall efficiency of the process.